This is a randomized, double blind treatment trial of the putative neuroprotective effects of allopurinol on the developing brain exposed to hypoxic-ischemic insults. Infants undergoing cardiac surgery which requires circulatory arrest are treated with either allopurinol or a placebo prior to surgery. The major outcome measures are acute encephalopathy, (including coma and seizures), and death. Infants undergo neurological examination and developmental assessment in follow-up visits up to two years of age. It has long been known that hypoxic-ischemia is a potent cause of cell death and tissue necrosis. The exact biochemistry of injury is still under intense investigation. One of the most fascinating discoveries in this line of research is that aberrant oxygen metabolism plays a role in tissue injury. There is a growing body of evidence which has identified oxygen free radical formation as an important biochemical cascade operative in the ultimate production of cellular insult. This model serves to provide a potential therapeutic mechanism of intervention that might prevent or ameliorate the consequences of hypoxia reperfusion injury. Allopurinol may prevent the formation of free radicals in post ischemic tissues. To date 251 patients have been enrolled in the clinical trial. An interim statistical analysis of primary outcomes is planned for spring 1996.